In a boat that moves by use of a boat propulsion engine which comprises an outboard motor, the level of the stern drops and sinks into the water, and the bow rises and is tilted upward when, for example, the boat is at a standstill and when the boat begins to move. Since the hull therefore begins to move in a tilted state, the water resistance is considerable when traveling starts, and adequate boat speed cannot be obtained. The stern must rise upward a certain amount, and the orientation of the boat must become approximately horizontal in order to reach a certain level of speed. There is a problem in that time is required for the boat to approximate an orientation that is nearly horizontal, and the boat cannot smoothly accelerate.
An outboard engine that can improve the acceleration characteristics of a boat is disclosed in Japanese Patent Laid-Open Publication No. 5-319386 (JP-5-319386A) and Japanese Utility Model Laid-Open Publication No. 47-9194 (JP-UM-47-9194A).
In the outboard engine of the 5-319386 publication, an engine, a vertically disposed drive shaft and other drive components, and transmission components are covered by a vertical cowling. A propulsion casing is disposed below the lower cowling so as to provide vertical linkage. When the boat is at a standstill, a portion of the lower cowling is submerged, and when the boat is moving, only the propulsion casing is submerged.
In the outboard engine of the 47-9194 publication, the waterproof engine casing that covers the engine is formed having a size that is sufficient to provide flotation to the engine, and the engine is designed to float on the surface of the water.
In the outboard engine of the 5-319386 publication, however, a portion of the lower cowling that forms the engine room is structured to submerge, and it is therefore difficult to endow this structure with water tightness when the lower cowling is assembled. When water has furthermore flooded the engine room, it is difficult to drain the water, the movement of movable components is compromised by water and salt, and the components tend to corrode.
The outboard engine mounted on the stern moreover has a structure in which the engine is covered with an upper and lower cowling, an extension case is provided below the lower cowling, and a gear case is disposed under the extension case. Therefore, the driving noise of the engine passes through the case and the cowlings, and is released to the periphery as engine noise.
In the outboard motor of the 5-319386 publication, the buoyancy of the hull itself only prevents the stern from sinking when the boat accelerates from a standstill in particular, and the design does not provide for actively lifting the stern and quickly bringing the hull into a horizontal state.
Thus, the depth of the outboard engine when the boat is at a standstill or is accelerating must be reduced, the orientation of the hull must be made to rapidly transition to an approximately horizontal state during acceleration, and smooth acceleration must be achieved.